Recently, a compact disc digital audio system has been developed and in the limelight which records information signals, such as video or sound, in minute hollows (called the pits) on the disc and reads out and reproduces the recorded information by use of the ray, such as the laser ray, the disc being regulated to rotate at constant linear velocity so that the motor for rotatably driving the disc is adapted to be sequentially changeable of the number of rotations per minute following movement of a pickup.
Since the control signal controlling the motor is formed on the basis of data signal recorded on the disc, a method has been proposed which detects the synchronous signal with the longest data signal from the data signals reproduced by the pickup to thereby control the number of rotations of motor so that its time intervals are constant.
The above method includes a method of feeding to an integrated circuit the data signal detected from the disc to thereby detect the peak value by the longest synchronous signal, or that of picking up the longest synchronous signal of data signal by FM-detecting the data signal, the detection of peak value by the integrated circuit and pickup of synchronous signal by FM-detection being impossible during the start of motor, the pickup of data signal itself being almost impossible unless the disc is driven previously at constant linear velocity, thereby requiring another circuit which rotatably drives the motor by some way until it rotates at about the predetermined linear velocity during the starting. Therefore, the problem has been created in that the construction is complicated.